1. Field of the Invention
The present invention relates to a particle removing method for removing particles (dust) from a stage that holds a planar workpiece such as a semiconductor wafer, and a cleaning plate used according to the method.
2. Description of the Related Art
For processing or machining a planar workpiece (plate-like piece), normally, the plate-like piece is held on a stage having a flat placement surface. For example, during an electronic part manufacturing process for processing or machining a semiconductor wafer, the semiconductor wafer is locked or held on the stage according to a suction method by electrostatic force or vacuum suction method. The semiconductor wafer is held or locked on the stage during such processes as follows: a lithography process using a step-and-repeat photolithography system with demagnification or an electron-beam lithography system; a film handling process using a sputtering system, a deposition system, a chemical vapor deposition (CVD) system, and an etching system; an electric testing process for testing dies (chips) delineated on a semiconductor wafer using a prover and a tester; and a process for optically inspecting patterns drawn on a semiconductor wafer. Moreover, for manufacturing a panel for a liquid crystal display or plasma display, a glass substrate is locked or held on a stage in order to carry out various kinds of processing including patterning. A stage for holding a semiconductor wafer during the lithography process will be taken for instance in order to proceed to a further description. The present invention is not limited to this sort of stage. The present invention can be applied to any stage as long as the stage holds a planar workpiece (plate-like piece) for precise processing or machining.
FIG. 1 shows the basic structure of a wafer stage employed during the lithography process or the like. In FIG. 1, a stage 1 holds and locks a semiconductor wafer on the top thereof, and is placed on a Y-direction movable base 2. The Y-direction movable base 2 is supported to be movable in a Y direction along movement grooves bored in an X-direction movable base 3. The Y-direction movable base 2 moves in the Y direction when driven by a DC servomotor 5. The X-direction movable base 3 is supported to be movable in an X direction along movement grooves bored in a base 4, and moves in the X direction when driven by a DC servomotor 6. Owing to this mechanism, the stage 1 is movable in the two, X and Y, directions. In practice, the stage 1 is movable in Z directions or vertical directions. A laser interferometer or the like for precisely measuring a magnitude of movement is included in the stage but is omitted herein.
For holding and locking a placed semiconductor wafer, a vacuum suction method or a suction method by electrostatic force is adopted. FIG. 2 shows the schematic structure of a vacuum suction mechanism included in the wafer stage. As shown in FIG. 2, a plurality of holes is bored in the top of the stage 1 and communicating with a connection port via an air path 11 formed inside the stage 1. The connection port is linked to a vacuum pump 15 by way of a hose 12 and an air valve 13. After a semiconductor wafer 100 is placed on the top of the stage 1, the air valve 13 is turned to select the vacuum pump 15. This causes the vacuum pump 15 to operate (the vacuum pump is connected to a vacuum chamber). The interior of the air path 11 is depressurized accordingly. The semiconductor wafer 10 is then sucked onto the top of the stage 1 and locked. For collecting the semiconductor wafer 100 from the stage 1, the air valve 13 is turned to select an exhaust path 14. Outside air is introduced into the air path 11. Consequently, the semiconductor wafer is freed from the vacuum suction mechanism. Thereafter, a vertical movement pin, which is not shown, formed on the stage 1 is moved upward. With the semiconductor wafer 100 thus moved upward, a wafer transportation mechanism receives the semiconductor wafer. For placing the semiconductor wafer 100 on the top of the stage 1, the vertical movement pin is moved upward. In this state, the wafer transportation mechanism places the semiconductor wafer 100 on the vertical movement pin. After the wafer transportation mechanism withdraws, the vertical movement pin is moved downward. Consequently, the semiconductor wafer 100 is placed on the stage.
The foregoing vacuum suction mechanism is widely adopted as a suction mechanism for wafer stages. However, in a system in which a semiconductor wafer and a stage are held in a vacuum (depressurized) environment, such as an electron-beam lithography system, the vacuum suction mechanism is unusable. A suction mechanism utilizing static electricity is adopted.
Semiconductor devices have been considerably downsized in recent years. The occurrence of microscopic particles (dust) during a semiconductor manufacturing process greatly affects a yield of semiconductor devices. In general, the semiconductor manufacturing process is carried out in a very clean environment, or more particularly, in a clean room. Especially a lithography process for manufacturing semiconductor devices is required to achieve highly precise processing and is therefore carried out in a clean room of the highest level of cleanness. However, even when the processing is carried out in such an environment, it is impossible to perfectly prevent occurrence of particles. A decrease in the yield caused by adhesion of particles onto the top of a semiconductor wafer has been discussed mainly in the past. A method of removing particles from the air circulated within a clean room using a filter or the like has been adopted in the past. The standard stipulating the degree of cleanness of a clean room describes the number of particles existing in the air.
However, particles not floating in the air but adhering to the surface of a stage may be scattered into the air during placement or collection of a semiconductor wafer. It is highly possible that the particles may adhere to the surface of another semiconductor wafer and become a factor of the decrease in the yield. It is relatively small particles that may be scattered into the air during placement or collection of a semiconductor wafer. Such particles are scattered into the air during collection of a semiconductor wafer, and a semiconductor wafer to be exposed next is then supplied immediately thereafter. There is therefore a high possibility that the scattered particles may adhere to the surface of the next semiconductor wafer. This greatly affects the yield.
Moreover, relatively large particles are less likely to scatter than small particles. However, when a semiconductor wafer is placed on the surface of a stage to which the large particles adhere, if the semiconductor wafer is duly sucked, the flatness of the semiconductor wafer deteriorates due to the particles. FIG. 3A and FIG. 3B show this condition. When the semiconductor wafer 100 is placed on the stage 1 having a particle 21 and is sucked as shown in FIG. 3A, the part of the semiconductor wafer lying on the particle 21 is higher than the other part thereof. This leads to deteriorated flatness. Besides, the semiconductor wafer 100 is deformed around the particle 21 during suction. If the particle is small enough, the deterioration in the flatness of the semiconductor wafer or the deformation thereof is negligible. For this reason, almost no measures have been taken to remove particles adhering to the surface of a stage.
However, semiconductor devices have been getting smaller in recent years. A numerical aperture (NA) offered by the step-and-repeat photolithography system with demagnification has reached 0.5 or more. Since a depth of focus is very small (shallow), even a small decrease in the flatness causes trouble. Moreover, the electron-beam lithography system offers a larger depth of focus than the step-and-repeat photolithography system with demagnification. The decrease in the flatness is manifested as a positional deviation. Besides, patterns drawn by the electron-beam lithography system are finer than those drawn by the step-and-repeat photolithography system with demagnification. Therefore, the effect of particles adhering to a stage cannot be ignored any longer.
As mentioned above, almost no measures have been taken to remove particles adhering to the surface of a stage. In general, for removing particles adhering to the surface of an apparatus, the surface is washed using pure water or any other fluid, wiped out using a cleaning cloth, or blown out with an air flow. However, since the stage is mounted on an apparatus, it cannot be washed using a fluid. If the surface of the stage were wiped out using a cleaning cloth or blown out with an air blow, particles would be scattered within the apparatus. This work causes new particles to appear.
In particular, as far as an electron-beam lithography system or a film handling system such as the sputtering system, deposition system, CVD system, or etching system is concerned, the stage must be placed in a depressurized state or in a predetermined gaseous atmosphere. When an attempt is made to remove particles from the stage according to the foregoing method, the atmosphere in which the stage is placed must be changed to the air. This leads to a decrease in the operating rate of the system, and causes adhesion of new particles to the stage.
Particles are attributable to contact between a semiconductor wafer and the surface of the stage or contact between the vertical movement pin and the semiconductor wafer, and grow from the surfaces of the stage and semiconductor wafer. Moreover, particles falling off from a semiconductor wafer supplied from outside a system for processing or particles floating in the atmosphere of the system are thought to adhere to semiconductor wafers. Particles adhering to the surface of the stage may be transported out of a system while adhering to the surface of a semiconductor wafer during supply or collection of the semiconductor wafer, or may be scattered into the other portion of the system or into the air. Unless particles are removed by performing thorough cleaning, particles would accumulate and the number of particles would increase. As mentioned above, once a stage is mounted in a system, it is hard to clean the stage. It is by no means easy to remove particles from the stage.
As mentioned above, the removed of particles on a stage has not been discussed very seriously. It is hard to remove particles. However, along with the further advancement of a microprocessing technology, removing particles from a stage has become important for improving the yield. An object of the present invention is to establish a method of readily removing particles from a stage and to improve a yield by removing particles without substantially causing a decrease in an operating rate according to the method.
For accomplishing the above object, a method of removing particles from a stage according to the present invention is such that a resin film is placed on a stage and then collected therefrom after particles adhere thereto. Thus, the particles are removed from the stage.
In other words, a method of removing particles from a stage according to the present invention is a stage particle removing method for removing particles from a stage that holds a planar workpiece when implemented in a system for processing or machining the planar workpiece. The method is characterized in that a resin film is placed on the stage and then collected from the stage.
The resin film is less hard and more elastic than a ceramic made into the stage or a metal. More particles are likely to adhere to the resin film than to the surface of the stage. Therefore, when the resin film is placed on the stage to which particles adhere, the particles stick to the resin film. When the resin film is collected, almost all the particles are collected while adhering to the resin film. According to this method, since particles are collected while adhering to the resin film, there is no fear that the particles may be scattered into the air or stuck to any other part.
The resin film may be realized as a planar plate formed with a resin film alone. Preferably, the resin film should be coated over at least one surface of a planar workpiece such as a semiconductor wafer or glass substrate, and the planar workpiece should then be placed with the resin film side thereof in contact with the stage. Moreover, the resin film may not be coated over the planar workpiece itself but may be coated over a dedicated planar piece that is shaped similarly to the planar workpiece, for example, a thin metallic plate that is quite smooth. In this case, particles can be removed from the stage by utilizing the same supply/discharge route as that along which the planar workpiece is supplied or discharged. Particles can therefore be removed efficiently and quickly. Moreover, the used resin film should preferably be peeled off from the planar workpiece or dedicated planar piece, and the planar workpiece or dedicated planar piece should be reused by being recoated with another resin film For enabling automatic transportation of the resin film or removing particles highly efficiently, the resin film must be very flat. A difference in film thickness should preferably be 100 xcexcm or less. More preferably, the difference should be 20 xcexcm or less. Herein, the resin film or the plate-like piece coated with the resin film shall be referred to as a cleaning plate.
The present invention can be implemented in any system as long as the system includes a stage for holding and locking a planar workpiece. When implemented especially in a semiconductor manufacturing system for processing or machining a semiconductor wafer that is susceptible to particles, the present invention proves most advantageous.
The hardness of the resin film should preferably be of a level permitting the resin film to deform readily when meeting a particle and permitting the particle to adhere to the resin film.
The resin film is produced by, for example, applying a resin directly to the planar workpiece or dedicated planar piece according to a screen printing method, and then hardening it. Otherwise, a resin is molded into the state of a film according to any of various molding methods.
Before use, the resin film must be retained in a state in which few particles adhere to the resin film. The resin film must therefore be cleaned using a solvent such as alcohol, acetone, or isopropyl alcohol. For this reason, for example, an acrylic rubber, a butadiene rubber, a styrene rubber, a nitryl rubber, or a silicone resin such as a silicone elastomer or a silicone rubber that is durable to the solvent should preferably be adopted. A crosslinked (addition) resin and a condensation resin are known as silicone rubbers (elastomers). Preferably, the silicone rubber of the crosslinked (addition) resin that is little contracted and evaporated when hardened should be adopted.
As mentioned above, the resin film may be produced by applying a resin directly to a wafer or the like according to a screen printing technique or the like and then hardening it. Alternatively, a resin film produced as an independent film may be bonded to a wafer using an adhesive afterward. The adhesive may be a silicone adhesive or an epoxy adhesive but is not restricted to any specific one. Any adhesive will do as long as the adhesive exerts a predetermined bonding strength.
The film should preferably contain a filler or an additive such as a pigment. For fear of secondary contamination by the film, the filler should be perfectly covered with a resin, or in other words, the filler should not be exposed, by itself, on the surface of the film.
For preventing the filler from being exposed, the surface of the filler is finished using a coupling material or a primer. Thus, the wettability of the surface of the filler in contact with a resin should be improved.
When a collected resin film is cleaned using a solvent, if the resin film can be restored to the original state in which the resin film has few particles, the resin film is cleaned and then reused.
Assuming that the stage has a vacuum suction mechanism or a suction mechanism by electrostatic force, after the resin film is placed on the stage, the resin film should preferably be sucked, freed, and then collected. Thus, particles can be reliably stuck to the resin film. As mentioned above, the stage included in the electron-beam lithography system may be placed in a depressurized environment. In this case, the suction mechanism by electrostatic force is adopted.